Optical modulator

Abstract

An exemplary optical modulator (100) according to the present disclosure is provided with: a pair of parallel Mach-Zehnder-type arm waveguides (114-1, 114-2) that are provided on a ferroelectric crystal (103) having an electro-optic effect and have mutually opposite polarization directions; and a pair of signal electrodes (122-1, 122-2) disposed at positions with both the arm waveguides (114-1, 114-2) interposed therebetween in a width direction.

Description

Optical Modulator

[0001] The present disclosure relates to optical modulators.

[0002] Thin film LN (LiNbO ３ Mach-Zehnder (MZ) optical modulators, which utilize the electro-optic effect and are typified by the MZ modulator, are advantageous in widening the modulation bandwidth, and are therefore the subject of active technical research. Such optical modulators using the electro-optic effect are often realized as single-phase drive optical modulators.

[0003] On the other hand, for driver circuits such as driver ICs (Integrated Circuits) for driving optical modulators, a differential drive design is more advantageous than a single-phase drive design in terms of the increased power consumption and reduced voltage resistance that accompany wider bandwidths. One method for driving an MZ optical modulator using the electro-optic effect with a differential drive driver IC is described in, for example, Patent Document 1.

[0004] US Patent Application Publication No. 2023 / 0107837

[0005] However, there is room for improvement in the differential drive configuration of an optical modulator using the electro-optic effect in terms of broadening the bandwidth and / or reducing power consumption.

[0006] An exemplary object of the present disclosure is to provide an optical modulator that contributes to wider bandwidth and / or lower power consumption.

[0007] Therefore, an optical modulator according to one aspect of the present disclosure is provided on a ferroelectric crystal having an electro-optic effect, and includes a pair of parallel Mach-Zehnder arm waveguides having polarizations in opposite directions, and a pair of signal electrodes arranged at positions sandwiching both of the arm waveguides in the width direction.

[0008] FIG. 11 is a top view showing an exemplary configuration of an optical modulator according to a first embodiment. FIG. 12 is a cross-sectional view taken along the line A-A' in FIG. 1. FIG. 13 is a cross-sectional view taken along the line A-A' as a modification of FIG. 2. FIG. 14 is a cross-sectional view schematically showing an electric field applied between signal electrodes in the first embodiment. FIG. 15 is a top view showing an exemplary configuration of an optical modulator according to a second embodiment. FIG. 16 is a cross-sectional view taken along the line B-B' in FIG. 5. FIG. 17 is a top view showing an exemplary configuration of an optical modulator according to a third embodiment. FIG. 18 is a cross-sectional view taken along the line C-C' in FIG. 7. FIG. 19 is a top view showing an exemplary configuration of an optical modulator according to a fourth embodiment. FIG. 20 is a top view showing an exemplary configuration of an optical modulator according to a fifth embodiment. FIG. 21 is a cross-sectional view taken along the line D-D' in FIG. 10. FIG. 22 is a cross-sectional view showing an exemplary differential drive configuration of an MZ optical modulator using the electro-optic effect.

[0009] Hereinafter, embodiments will be described in detail with reference to the drawings. However, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims. Furthermore, more detailed descriptions than necessary may be omitted. For example, detailed descriptions of well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0010] Furthermore, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships or ratios of elements may differ from reality. The drawings may also include portions in which the dimensional relationships or ratios differ. When numerical values ​​are used in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0011] <Overview> Fig. 12 is a cross-sectional view showing an exemplary differential drive configuration of an MZ optical modulator 1200 using the electro-optic effect. In Fig. 12, the X axis defines the height (or thickness) direction of the MZ optical modulator 1200, the Y axis defines the length direction of the MZ optical modulator 1200 and the propagation direction of the light and electrical signals, and the Z axis direction defines the width direction of the MZ optical modulator 1200.

[0012] As shown in FIG. 12, the MZ optical modulator 1200 includes a substrate (e.g., a Si substrate) 1202 and a lower clad layer (e.g., SiO ２ The optical waveguide includes a cladding layer 1204 and an LN layer 1206 disposed on the cladding layer 1204 .

[0013] The LN layer 1206 is, for example, an X-cut layer of an LN crystal, which is an example of a ferroelectric crystal. The LN layer 1206 is provided with a pair (two) of mesa-shaped (or rib-shaped) optical waveguides (which may also be referred to as “arm waveguides” hereinafter) 1261 and 1263 that are parallel to the Y-axis direction.

[0014] The arm waveguides 1261 and 1263 are each provided with a cladding layer (e.g., SiO ２ By covering the surface with a layer (or a thin film), it functions as a waveguide core with a relatively high refractive index, and confines and guides the incident light within the waveguide core.

[0015] In order to optically modulate the propagating light in the arm waveguides 1261 and 1263 using a differential driving electrical signal, for example, a pair of a ground electrode (G) and a signal electrode (S) is arranged on both sides of each arm waveguide 1261 and 1263 in the Z-axis direction.

[0016] 12, for one arm waveguide 1261, a ground electrode 1282 is arranged on the outer side in the Z-axis direction (width direction) and a signal electrode 1284 is arranged on the inner side in the width direction. Similarly, for the other arm waveguide 1263, a ground electrode 1288 is arranged on the outer side in the width direction and a signal electrode 1286 is arranged on the inner side in the width direction.

[0017] Therefore, the electrode arrangement in the Z-axis direction for the pair of optical waveguides 1261 and 1263 is the following arrangement: ground (G) electrode 1282 - signal (S) electrode 1284 - signal (S) electrode 1286 - ground (G) electrode 1288. For convenience, this electrode arrangement may be referred to below as the "GSSG arrangement" or "GSSG configuration."

[0018] 12, reference numeral 1260 denotes a region of the LN layer 1206 including an arm waveguide 1261, which is sandwiched between the ground electrode 1282 and the signal electrode 1284 in the Z-axis direction. Similarly, reference numeral 1262 denotes a region of the LN layer 1206 including an arm waveguide 1263, which is sandwiched between the ground electrode 1288 and the signal electrode 1286. For convenience, the regions 1260 and 1262 may be referred to below as "arm waveguide regions 1260 and 1262," respectively.

[0019] As shown in FIG. 12, when a differential drive voltage is applied between electrodes 1282-1284 and between electrodes 1286-1288, electric fields (or electric fields) in the same direction in the Z-axis direction are applied to arm waveguides 1261 and 1263 individually or independently.

[0020] Here, to achieve push-pull modulation, for example, the polarity (spontaneous polarization of LN) of one of the pair of arm waveguide regions 1260 and 1262 (e.g., arm waveguide region 1262) is reversed relative to the other arm waveguide region 1260. The region in which the spontaneous polarization is reversed may be referred to as a "polarization inversion region."

[0021] When an electric field is applied in the same direction in the Z-axis direction to the arm waveguide region 1262 of the polarization inversion region and the arm waveguide region 1260 of the non-polarization inversion region, the sign of the refractive index change is reversed between the two regions 1262-1260, thereby achieving push-pull modulation operation.

[0022] However, in the GSSG configuration illustrated in FIG. 12, an electric field can also be generated in the region between the signal electrodes 1284-1286 (between SS) where the arm waveguides 1261 or 1263 are not arranged and do not contribute to optical modulation (hereinafter referred to as the "non-modulation region").

[0023] Therefore, the parasitic capacitance of the signal electrodes 1284 and 1286, which are high-frequency lines, increases, and the characteristic impedance may decrease. As a result, the bandwidth characteristics of the optical modulation may deteriorate. Therefore, for example, it has been considered to suppress the coupling of electric fields between the SSs, which are non-modulation regions, by sufficiently widening the arrangement interval (gap) between the SSs in the GSSG configuration.

[0024] However, increasing the gap between the SSs in the Z-axis direction not only increases the element width but also increases the optical propagation loss due to the unnecessary expansion of the optical circuit, so it is unrealistic to move them far enough apart that the coupling between them can be ignored.

[0025] Furthermore, in this configuration, there is a limit to how narrow the electrode gap can be to apply a high electric field to the core: the metal electrodes and the arm waveguides must be separated by a distance that prevents the optical propagation field from interacting with the metal in order to suppress the increase in guided light loss due to metal absorption, and it is therefore difficult to narrow the electrode gap any further.

[0026] Therefore, the gaps (GAP#3) between the arm waveguide 1261 and the ground electrode 1282 and between the arm waveguide 1261 and the ground electrode 1282 and between the arm waveguide 1261 and the signal electrode 1284 may be set to, for example, about 3 micrometers (μm) or more. Similarly, the gaps between the arm waveguide 1263 and the ground electrode 1288 and between the arm waveguide 1263 and the signal electrode 1286 may be set to, for example, about 3 μm or more.

[0027] Assuming that the width of each of the arm waveguides 1261 and 1263 in the Z-axis direction is approximately 3 μm, in FIG. 12, the total inter-electrode (inter-GS) gap (GAP#1 and GAP#2) that contributes to optical modulation is at least approximately 18 μm.

[0028] As a result, in the MZ optical modulator 1200 with the GSSG configuration, it is difficult to simultaneously reduce the parasitic capacitance, which is determined depending on the inter-electrode gap, and improve the efficiency of the electric field strength applied to the arm waveguide.

[0029] Therefore, the following describes several embodiments that can simultaneously reduce the parasitic capacitance and improve the efficiency of the electric field strength for the arm waveguides by, for example, suppressing the unnecessary spread of the electric field into the non-modulation region and narrowing the inter-electrode gap. Some of the embodiments can improve the performance of the optical modulator (for example, broadening the bandwidth of the optical modulation and / or reducing the drive voltage).

[0030] <Embodiment 1> Fig. 1 is a top view showing an exemplary configuration of an optical modulator 100 according to embodiment 1, and Fig. 2 is a cross-sectional view taken along the line A-A' in Fig. 1. In Fig. 1 and Fig. 2, the X-axis defines the height (or thickness) direction of the optical modulator 100, the Y-axis defines the length direction of the optical modulator 100, which is the propagation direction of light and electrical signals, and the Z-axis defines the width direction of the optical modulator 100. This correspondence between the three axes is similar in other embodiments.

[0031] The optical modulator 100 illustrated in FIGS. 1 and 2 is an MZ optical modulator that utilizes the electro-optic effect, and includes, for example, an input waveguide 110, an input optical coupler 112, optical waveguides 114-1 and 114-2, an output optical coupler 116, and an output waveguide 118.

[0032] The input waveguide 110 guides the input light to the input optical coupler 112, which splits the input light from the input waveguide 110 into two beams, outputting one beam to the optical waveguide 114-1 and the other beam to the optical waveguide 114-2.

[0033] The optical waveguides 114-1 and 114-2 constitute a pair of MZ-type arm waveguides that are parallel to the Y-axis direction in the YZ plane. In the following description, when the arm waveguides 114-1 and 114-2 are not to be distinguished from each other, they may be abbreviated as "arm waveguide 114."

[0034] The arm waveguides 114-1 and 114-2 guide the light input from the input optical coupler 112 in the Y-axis direction. The output optical coupler 116 combines the output lights of the arm waveguides 114-1 and 114-2 and outputs the combined light to the output waveguide 118.

[0035] The relative phases of the light propagating through the arm waveguides 114-1 and 114-2 are controlled by the electro-optic effect, thereby controlling the interference state of the light in the output optical coupler 116, and modulated light according to the phase control is output from the output optical coupler 116. The output waveguide 118 guides the modulated light from the output optical coupler 116 and outputs it.

[0036] Therefore, each of the input waveguide 110, the input optical coupler 112, the pair of arm waveguides 114, the output optical coupler 116, and the output waveguide 118 may be made of a material having an electro-optic effect, such as LN, as a non-limiting example.

[0037] For phase control, a pair of signal (S) electrodes 122-1 and 122-2 extending along the extension direction of the arm waveguides 114-1 and 114-2 are provided on the outer sides of the arm waveguides 114-1 and 114-2 in the Z-axis direction, respectively.

[0038] In other words, both of the pair (two) arm waveguides 114-1 and 114-2 are arranged between the signal electrodes 122-1 and 122-2 in the Z-axis direction. This electrode arrangement is significantly different from the GSSG configuration in which the arm waveguides 1261 and 1263 are individually sandwiched between two electrodes in the Z-axis direction, as illustrated in Fig. 12. Note that when the signal electrodes 122-1 and 122-2 are not to be distinguished from each other, they may be abbreviated as "signal electrode 122."

[0039] Each of the signal electrodes 122 may be, for example, a coplanar strip line and may be made of a metal (or conductive) material such as aluminum (Al) or gold (Au). Titanium (Ti), for example, may be used to bond the signal electrodes 122 to the underlying layer (e.g., the upper clad layer 104 in FIG. 2 or the LN layer 103 in FIG. 3). Note that the "signal electrode" is an electrode to which a high-frequency electrical signal such as an RF signal is applied, and may also be referred to as a "high-frequency electrode" or a "traveling-wave electrode."

[0040] 1, for example, a differential drive driver 151 for applying a differential drive voltage between the signal electrodes 122 is electrically connected to one end of each of the signal electrodes 122 in the Y-axis direction. For example, a termination resistor 153 is electrically connected to the other end of each of the signal electrodes 122 in the Y-axis direction. Note that a pair of signal electrodes (122) to which a differential drive voltage is applied may be referred to as "differential signal electrodes."

[0041] Furthermore, for push-pull modulation operation, a region of one of the pair of arm waveguides 114 (for example, arm waveguide 114-1) forms a polarization inversion region 134 in which the polarity (spontaneous polarization of LN) is inverted relative to the region of the other arm waveguide 114-2.

[0042] Next, the cross-sectional structure of the MZ optical modulator 100 will be described with reference to the cross-sectional view taken along the line A-A' in Fig. 2. As shown in Fig. 2, the MZ optical modulator 100 includes, for example, a substrate (e.g., a Si substrate) 101, a lower cladding layer 102 provided on the substrate 101, an LN layer 103 provided on the lower cladding layer 102, and an upper cladding layer 104 provided on the LN layer 103.

[0043] The LN layer 103 is a layer of an LN crystal, which is an example of a ferroelectric crystal, and is, for example, an X-cut LN crystal layer in which the electro-optic effect is prominent for an electric field component parallel to the substrate 101. The LN layer 103 may be replaced with, for example, a layer of a polymer-based electro-optic material (EO polymer).

[0044] An input waveguide 110, an input optical coupler 112, a pair of arm waveguides 114, an output optical coupler 116, and an output waveguide 118 are each formed by, for example, patterning a waveguide pattern on the LN layer 103. Therefore, each of the arm waveguides 114 functions as an optical modulation waveguide extending along the Y-axis direction on a plane orientation equivalent to an X-cut.

[0045] The optical waveguide may be processed by dry etching or chemical wet etching. The mask material for the waveguide may be SiO. ２ It does not matter whether the material is a dielectric such as a silicon dioxide (SiO2), a metal such as chromium (Cr), or a photoresist. Furthermore, the mesa depth of the etched waveguide is merely a design matter for optimizing the device capacitance, propagation loss, etc., and is not limited to a specific mesa depth.

[0046] 2 shows an example in which a pair of arm waveguides 114-1 and 114-2 are provided on the LN layer 103. Furthermore, a partial region 134 of the LN layer 103 in which one arm waveguide 114-1 is provided constitutes a "polarization inversion region" in which the spontaneous polarization is inverted relative to the other arm waveguide 114-2.

[0047] The polarization inversion region 134 may have a width in the Z-axis direction that is wider than the width of the arm waveguide 114. As a non-limiting example, the width in the Z-axis direction of the arm waveguide 114 may be about 3 μm, and the width in the Z-axis direction of the polarization inversion region 134 may be about 5 μm.

[0048] Each of the input waveguide 110, the input optical coupler 112, the pair of arm waveguides 114, the output optical coupler 116, and the output waveguide 118 is made of a material having a refractive index lower than that of the LN layer 103 (e.g., SiO ２ ) is surrounded by a lower cladding layer 102 and an upper cladding layer 104 .

[0049] In other words, each of the input waveguide 110, the input optical coupler 112, the pair of arm waveguides 114, the output optical coupler 116, and the output waveguide 118 is embedded in the cladding material.

[0050] With this structure, light input to the MZ optical modulator 100 is confined and propagated in each of the input waveguide 110, the input optical coupler 112, the pair of arm waveguides 114, the output optical coupler 116, and the output waveguide 118. The lower cladding layer 102 and the upper cladding layer 104 may have the same composition or different compositions.

[0051] Each of the cladding layers 102 and 104 may have an appropriate thickness in the X-axis direction in consideration of the optical characteristics and / or high frequency characteristics, and may have a refractive index lower than that of the LN layer 103, for example.

[0052] Furthermore, the thickness of the upper cladding layer 104 may be, for example, a thin film layer thinner than the thickness of the lower cladding layer 102. As a non-limiting example, the thickness of the upper cladding layer 104 may be a value in the range of about 100 to 500 nanometers (nm) (e.g., about 300 nm).

[0053] Depending on the gap between the signal electrode 122 and the arm waveguide 114 in the Z-axis direction, the upper clad layer 104 may not be provided below each of the signal electrodes 122, as shown in Fig. 3. For example, the upper clad layer 104 may be provided so as to cover each of the arm waveguides 114, limited to a region sandwiched between the signal electrodes 122-1 and 122-2 in the Z-axis direction.

[0054] As illustrated in FIGS. 2 and 3, the upper clad layer 104 may or may not be present below each signal electrode 122, and this is the same as in other embodiments described later.

[0055] In the MZ optical modulator 100 configured as described above, when a differential drive voltage (+S or -S) is applied between the signal electrodes 122 by the differential drive driver 151, a unidirectional electric field is applied to both arm waveguides 114 sandwiched between the signal electrodes 122 in the Z-axis direction.

[0056] 4, when a negative voltage signal (-S) is applied to the signal electrode 122-1 and a positive voltage signal (+S) is applied to the signal electrode 122-2, an electric field is generated from the signal electrode 122-2 toward the signal electrode 122-1. This electric field is applied in common to both the arm waveguides 114-1 and 114-2.

[0057] As described above, the arm waveguides 114-1 and 114-2 have a relationship in which their spontaneous polarizations are inverted relative to each other, and therefore, as schematically shown by the block arrows in the Z-axis direction in Fig. 4, the signs of the refractive index changes with respect to the change in the electric field commonly applied to each arm waveguide 114 are inverted relative to each other. Therefore, the phase changes of the light propagating through each arm waveguide 114 are inverted relative to each other, thereby realizing push-pull modulation operation.

[0058] Here, in the MZ optical modulator 100 of the first embodiment, since there is no electrode between the arm waveguides 114-1 and 114-2 in the Z-axis direction, the degree of freedom in designing the gap provided between the pair of arm waveguides 114 is improved.

[0059] For example, even if the gap between the arm waveguides 114 is narrower than the configuration illustrated in Fig. 12, there is no deterioration in characteristics such as an increase in parasitic capacitance depending on the inter-electrode gap or an increase in waveguide loss due to light absorption by the metal electrodes. Therefore, it is easy to increase the electric field strength applied to each arm waveguide 114 by narrowing the gap between the arm waveguides 114 while suppressing deterioration in characteristics.

[0060] As a non-limiting example, assume that the gap between the signal electrode and the arm waveguide, the width of each arm waveguide (waveguide width), and the gap between the arm waveguides are all 3 μm in the Z-axis direction, and that the input electrical signal applied to the signal electrode is ±0.5 volts (V), i.e., the differential drive voltage is 1.0 Vppd (peak-to-peak differential).

[0061] 12, the voltage applied between the ground electrode and the signal electrode (between GS) is 0.5 V, and each of the inter-electrode gaps #1 and #2 is 3.0 μm×3=9.0 μm. Therefore, the electric field strength (E) applied to each of the arm waveguides 1261 and 1263 is E=0.5 / 9.0≈0.056 (V / μm).

[0062] In contrast, in the first embodiment, the voltage applied between the signal electrodes 122 (between S and S) is 1.0 Vppd, and the gap (GAP#4) between the signal electrodes 122 is, in FIG. 4 , GAP#4 = GAP#1 + GAP#2 + GAP#3 + (2 × arm waveguide width) = 3.0 μm × 5 = 15 μm.

[0063] Therefore, the electric field strength E applied to each arm waveguide 114 is E = 1.0 / 15 ≈ 0.067 (V / μm), which is about 1.2 times stronger than the GSSG configuration illustrated in Fig. 12. This makes it possible to reduce the differential drive voltage required to obtain a desired electric field strength, which contributes to lowering the power consumption of the differential driver 151, for example.

[0064] As described above, according to the MZ optical modulator 100 of the first embodiment, it is possible to narrow the gap between the arm waveguides 114 while suppressing deterioration of characteristics depending on the gap between electrodes in the Z-axis direction, which contributes to performance improvements such as broadening the bandwidth of optical modulation and / or reducing the driving voltage.

[0065] Second Embodiment FIG. 5 is a top view showing an exemplary configuration of an optical modulator 200 according to a second embodiment, and FIG. 6 is a cross-sectional view taken along the line BB' in FIG.

[0066] 5 and 6 differs from the MZ optical modulator 100 of the first embodiment in that a groove (or trench) 136 extending along the Y-axis direction is provided in the LN layer 103 between the arm waveguides 114-1 and 114-2. The other components of the optical modulator 200 may be understood to be the same as or similar to those of the MZ optical modulator 100 of the first embodiment.

[0067] The length of the groove 136 in the Y-axis direction may be, for example, equal to or greater than the length of the arm waveguides 114-1 and 114-2 that form the modulation region. As a non-limiting example, the width of the groove 136 in the Z-axis direction may be about 1 μm, and the depth of the groove 136 in the X-axis direction may be about half the thickness of the LN layer 103, for example, 300 nm / 2=150 nm.

[0068] By providing the groove 136 between the arm waveguides 114, it is possible to suppress mode coupling between the arm waveguides 114. Therefore, for example, compared with the MZ optical modulator 100 of the first embodiment, it is possible to further narrow the gap between the arm waveguides 114 (GAP #2 in FIG. 6).

[0069] For example, it becomes possible to reduce the gap between the signal electrodes 122 in the Z-axis direction (GAP #4 in FIG. 6), and as a result, it becomes possible to increase the electric field strength applied to each of the arm waveguides 114, which contributes to reducing the differential drive voltage.

[0070] Third Embodiment FIG. 7 is a top view showing an exemplary configuration of an optical modulator 300 according to a third embodiment, and FIG. 8 is a cross-sectional view taken along the line CC' in FIG.

[0071] The MZ optical modulator 300 illustrated in Figures 7 and 8 differs from the MZ optical modulator 100 of embodiment 1 in that the widths of the arm waveguides 114-1 and 114-2 in the Z-axis direction change (or differ) in the light propagation direction (Y-axis direction).

[0072] For example, the first arm waveguide 114-1 has a structure in which the width changes from a first width W1 to a second width W2 that is narrower (or thinner) than the first width at a midpoint in the light propagation direction. In other words, the arm waveguide 114-1 has a partial waveguide 141 with a width W1 and a partial waveguide 142 with a width W2 that is thinner than the width W1.

[0073] In contrast to this, the second arm waveguide 114-2 has a structure in which its width changes from the second width W2 to the first width W1 in the light propagation direction, inversely to the change in width of the first arm waveguide 114-1. In other words, the arm waveguide 114-2 has a partial waveguide 143 with a width W2 and a partial waveguide 144 with a width W1.

[0074] In this way, the pair of parallel arm waveguides 114-1 and 114-2 have a structure in which the widths thereof change in the light propagation direction, and the manner of the change is interchangeable between the arm waveguides 114-1 and 114-2. In other words, the arm waveguides 114 have a structure in which the widths of the arm waveguides 114 change complementarily between each other in the light propagation direction.

[0075] The portions where the width changes (or transitions) between the partial waveguides 141 and 142 and between the partial waveguides 143 and 144 may be formed, for example, in a smooth tapered shape to minimize waveguide loss.

[0076] Furthermore, in the following description, for convenience, the partial waveguides 141 and 144 with width W1 may be referred to as "wide waveguides 141 and 144," respectively, and the partial waveguides 142 and 143 with width W2 may be referred to as "narrow waveguides 142 and 143," respectively.

[0077] In each of the arm waveguides 114, the length ratio in the light propagation direction between the “wide waveguide” (141 or 144) and the “narrow waveguide” (142 or 143) may be 1:1 or a ratio different from 1:1.

[0078] In the MZ optical modulator 300 having the arm waveguide 114 having the above-described structure, a difference occurs in the propagation speed (in other words, the amount of phase change) of the light translating through the arm waveguides 114-1 and 114-2.

[0079] For example, since the speed of propagating light can be slower as the waveguide width becomes wider, the propagating light in the wide waveguide 141 in the arm waveguide 114-1 can be delayed in phase with respect to the propagating light in the narrow waveguide 143 in the arm waveguide 114-2. Similarly, the propagating light in the wide waveguide 144 in the arm waveguide 114-2 can be delayed in phase with respect to the propagating light in the narrow waveguide 142 in the arm waveguide 114-1.

[0080] Here, the widths of the arm waveguides 114 vary complementarily between the arm waveguides 114 in the light propagation direction, so that the ratio of the lengths of the “wide waveguide” and the “narrow waveguide” in one arm waveguide 114 matches between the arm waveguides 114.

[0081] Therefore, in a state where no electric field is applied to each arm waveguide 114, the light propagating through each arm waveguide 114 undergoes phase changes of opposite signs over the propagation distance of each arm waveguide 114. Thus, zero-chirp modulation operation is achieved.

[0082] Therefore, for example, the gap between the arm waveguides 114 can be narrowed without causing deterioration in characteristics due to chirp, as in the first embodiment, and thereby the gap between the signal electrodes 122 can be narrowed, which contributes to performance improvements such as broadening the bandwidth of optical modulation and / or reducing the driving voltage.

[0083] The configurations illustrated in FIGS. 7 and 8 are examples in which one set of a “wide waveguide” and a “narrow waveguide” is provided in each of the arm waveguides 114, but multiple sets of a “wide waveguide” and a “narrow waveguide” may be repeatedly provided in the light propagation direction.

[0084] In other words, a plurality of sets of regions that are alternately divided into "wide waveguides" and "narrow waveguides" along the light propagation direction may be provided in each of the arm waveguides 114. Increasing the number of divided regions makes it easier to match (velocity match) the speed of the high-frequency signal in the signal electrode 122 with the speed of the propagating light in the arm waveguide 114.

[0085] Furthermore, the third embodiment may be implemented in combination with the second embodiment. For example, a groove 136 capable of suppressing mode coupling may be provided between arm waveguides 114 whose width in the Z-axis direction varies depending on the Y-axis position, as illustrated in Figures 5 and 6. By providing the groove 136, the third embodiment can achieve the same effects as the second embodiment.

[0086] 9 is a top view showing an exemplary configuration of an optical modulator 400 according to embodiment 4. The MZ optical modulator 400 shown in Fig. 9 may be understood to correspond to a modified example of embodiment 3, and has a configuration in which, for example, the signal electrodes 122 are partially offset in the Z-axis direction in a portion where the width of each arm waveguide 114 in the Z-axis direction changes.

[0087] For example, the signal electrode 122-1 for the first arm waveguide 114-1 is offset in a direction approaching the narrow waveguide 142 in the portion corresponding to the narrow waveguide 142 so that the separation distance in the Z-axis direction from the outer end of the wide waveguide 141 in the width direction is maintained.

[0088] Similarly, the signal electrode 122-2 for the second arm waveguide 114-2 is offset in the direction away from the wide waveguide 144 in the portion corresponding to the wide waveguide 144 so that the separation distance in the Z-axis direction from the outer end of the narrow waveguide 143 in the width direction is maintained.

[0089] The width of each of the signal electrodes 122-1 and 122-2 in the Z-axis direction may be constant, so that each of the signal electrodes 122 may have a portion that is smoothly curved in the Z-axis direction at the offset position (Y-axis position).

[0090] As described above, by partially offsetting each of the signal electrodes 122 so that the separation distance in the Z-axis direction is constant in accordance with the waveguide width of each of the arm waveguides 114, the electric field intensity applied to each of the parallel arm waveguides 114 can be stabilized to a constant value at the Y-axis position.

[0091] For example, the electric field generated between the signal electrodes 122 has the highest intensity at the metal edge portion of each signal electrode 122. Therefore, by making the distance between the metal edge portion and each arm waveguide 114 constant as described above, the electric field intensity applied to each arm waveguide 114 can be stabilized at a constant value.

[0092] Therefore, compared to the third embodiment, this contributes to improving the efficiency of the electric field intensity applied to each arm waveguide 114 .

[0093] As explained in the third embodiment, when each of the arm waveguides 114 is provided with a plurality of sets of regions that are alternately divided into "wide waveguides" and "narrow waveguides" along the light propagation direction, the signal electrode 122 may be partially offset for each divided region.

[0094] In the configuration illustrated in FIG. 9, a groove 136 capable of suppressing mode coupling may be provided between the arm waveguides 114, as illustrated in FIGS. 5 and 6 of the second embodiment.

[0095] Fifth Embodiment FIG. 10 is a top view showing an exemplary configuration of an optical modulator 500 according to a fifth embodiment, and FIG. 11 is a cross-sectional view taken along the line DD' in FIG.

[0096] 10 and 11 differs from the first embodiment in that each of the signal electrodes 122 is a capacitance-loaded traveling wave electrode, and that the groove 136 illustrated in the second embodiment is provided between the arm waveguides 114. The other components of the optical modulator 200 may be understood to be the same as or similar to the MZ optical modulator 100 of the first embodiment. Note that the groove 136 does not have to be provided in the fifth embodiment.

[0097] Each of the capacitively loaded traveling-wave electrodes 122 has a configuration in which a plurality of capacitive sub-electrodes 124 are discretely arranged in the Y-axis direction, protruding in a T-shape (however, not limited to a T-shape) in the Z-axis direction approaching the arm waveguide 114 in a top view. For convenience, each of the sub-electrodes 124 may be referred to as a "T-shaped electrode 124."

[0098] As shown in FIG. 11, in the Z-axis direction, both arm waveguides 114-1 and 114-2 are located between the T-shaped electrode 124 of signal electrode 122-1 and the T-shaped electrode 124 of signal electrode 122-2, and an electric field is applied in the region sandwiched between the T-shaped electrodes 124.

[0099] By using a capacitance-loaded signal electrode 122 having multiple T-shaped electrodes 124, it is possible to change (or adjust) the capacitance of the entire signal electrode 122 including the T-shaped electrodes 124, for example, by changing the area of ​​each T-shaped electrode 124. This improves the design freedom for capacitance (or characteristic impedance). By selecting the optimal capacitance, it is possible to achieve good high-frequency characteristics.

[0100] The fifth embodiment may be implemented in combination with the third or fourth embodiment. For example, in a mode in which the waveguide width of the arm waveguide 114 changes in the light propagation direction, a capacitively loaded signal electrode 122 may be used in which a plurality of T-shaped electrodes 124 are discretely arranged along the light propagation direction. In a combination of the fifth embodiment and the fourth embodiment, for example, the separation distance in the Z-axis direction can be flexibly set in units of the T-shaped electrodes 124 in accordance with the change in the waveguide width of the arm waveguide 114.

[0101] <Supplementary Note> In the above-described first to fifth embodiments, examples have been shown in which light propagates in the Y-axis direction in the X-cut LN layer 103. Alternatively, a configuration in which light propagates in the X-axis direction in the Y-cut LN layer may be employed. Furthermore, lithium tantalate (LiTaO ３ ) may also be used.

[0102] Furthermore, in the above-described embodiments 1 to 5, an SS configuration in which both arm waveguides 114 are sandwiched between a pair of signal (S) electrodes 122 in the Z-axis direction is exemplified, but the present disclosure is not limited to this.

[0103] For example, a GSSG configuration in which a ground electrode (G) is provided on the Z-axis direction outer side of each of a pair of signal electrodes 122 may be employed in any one or more of the first to fifth embodiments. When the GSSG configuration is employed, for example, the electric field strength between the pair of signal electrodes 122 (between SS) is relatively increased, and the electric field strength between the ground electrode and one signal electrode 122 (between GS) is relatively decreased, so that the GSSG configuration can also achieve the same effects as the SS configuration.

[0104] The term "connect" used in this disclosure may be read as "coupled." "Connected" or "coupled" may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, the term may also be understood to include an indirect "connection" or "coupling" where one or more intermediate elements are interposed between two elements that are "connected" or "coupled" to each other.

[0105] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0106] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0107] The present disclosure is useful, for example, in optical communication technology.

[0108] 100, 200, 300, 400, 500 Optical modulator (MZ optical modulator) 101 Substrate (Si substrate) 102 Lower cladding layer 103 LN layer 104 Upper cladding layer 110 Input waveguide 112 Input optical coupler 114-1, 114-2 Optical waveguide (arm waveguide) 116 Output optical coupler 118 Output waveguide 122-1, 122-2 Signal electrode 124 Sub-electrode 134 Polarization inversion region 136 Groove 141, 144 Partial waveguide (wide waveguide) 142, 143 Partial waveguide (narrow waveguide) 151 Differential drive driver 153 Termination resistor

Claims

1. An optical modulator comprising a pair of parallel Mach-Zehnder arm waveguides with opposite polarization directions provided on a ferroelectric crystal having an electro-optic effect, and a pair of signal electrodes positioned to sandwich both arm waveguides in the width direction.

2. An optical modulator according to claim 1, wherein said pair of signal electrodes are differential signal electrodes to which a differential drive voltage is applied, and each of said arm waveguides receives a common electric field between said differential signal electrodes.

3. An optical modulator according to claim 1, wherein a groove extending along the extension direction of said arm waveguides is provided in said ferroelectric crystal between said arm waveguides.

4. The optical modulator according to claim 1, wherein the widths of the arm waveguides change complementarily between the arm waveguides in the light propagation direction.

5. An optical modulator as claimed in claim 4, wherein the change in width is set so that, when no electric field is applied by the signal electrode, light propagating through each of the arm waveguides undergoes phase changes of opposite signs over the propagation distance in each of the arm waveguides.

6. An optical modulator according to claim 4, wherein each of said signal electrodes is partially offset in the width direction so that the distance between said signal electrodes and adjacent arm waveguides in said width direction is constant as said width changes.

7. An optical modulator according to claim 1, wherein each of said signal electrodes is a capacitively loaded electrode having a plurality of capacitive sub-electrodes discretely arranged along the extension direction of each of said arm waveguides.

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